1. Field of the Invention
The present invention relates to an image forming apparatus and, in particular, to a lock mechanism for a caster mounted to the bottom surface of an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a printer, a facsimile apparatus, and a copying machine, it is common practice to arrange in the lower portion of the apparatus a sheet feeding apparatus for supplying a sheet to an image forming portion of the image forming apparatus. As a result of the recent increase in the processing speed of an image forming apparatus, there is a demand for a large space for accommodating a large amount of sheets in the sheet feeding apparatus. When a sheet stacking portion for accommodating sheets increases in size, it becomes impossible to arrange the image forming apparatus on a table, so that the image forming apparatus is placed on the floor.
In order that the image forming apparatus placed on the floor may be allowed to easily move, a plurality of casters are mounted to the bottom surface of the image forming apparatus. Through the rotation of the wheels of the casters, it is possible for the user to move the image forming apparatus with a small force. However, when the wheels of the casters are always kept in a state in which they can rotate, it can happen that the image forming apparatus may be inadvertently allowed to move if, for example, an external force is applied or an earthquake or the like occurs. In view of this, at least a part of the casters is provided with a lock mechanism for locking the wheel. By locking the wheel by the lock mechanism after the image forming apparatus has been installed at a desired position, it is possible to fix the position of the image forming apparatus.
In a construction for locking the wheel, a lock lever is operated, and a protrusion provided on a lock member is caused to enter a plurality of recesses, holes, etc., formed in the wheel at equal peripheral intervals around a shaft on the inner side of the wheel, whereby the rotation of the wheel is stopped. Here, if the protrusion of the lock member and the recess of the wheel do not coincide with each other, it is impossible for the protrusion of the lock member to enter the recess, so that, when locking the wheel, it is necessary to rotate the wheel so as to set the protrusion of the lock member and a recess of the wheel in phase with each other.
In the case where the image forming apparatus is light in weight, when the lock lever is operated, it is possible to rotate the wheel with the force of the user. On the other hand, in the case where the image forming apparatus is heavy, it is rather difficult to generate through lock lever operation by the user a force large enough to overcome the friction between the wheel and the ground. Thus, in the case where the image forming apparatus is heavy, the user must directly rotate the wheel to match the protrusion of the lock member with the recess of the wheel, and then operate the lock lever to lock the wheel.
In this process, the protrusion of the lock member and the recess of the wheel are situated inside the wheel, so that they cannot be easily seen from the outside. Thus, when locking the wheel, the user appropriately rotates the wheel by hand to operate the lock lever. Since it is difficult to lock the wheel through a single rotational operation on the wheel, it is necessary to rotate the wheel several times to try the lock lever operation.
On the other hand, the caster is provided so as to be rotatable parallel to the bottom surface of the image forming apparatus, and, to achieve further stability in traveling, the rotation center of the caster and the grounding position of the wheel are deviated from each other. As a result, when the image forming apparatus is pushed, the caster rotates along the pushing direction, and the moving direction of the image forming apparatus and the rotational direction of the wheel coincide with each other, making it easier for the image forming apparatus to travel.
In this case, the caster grounding point changes depending on the rotation of the caster. Generally speaking, the caster position is so designed that the stability of the image forming apparatus in the installed state is secured independently of the rotational position of the caster. However, depending upon the caster position, there exist a most advantageous condition and a most disadvantageous condition with respect to stability.
That is, when the image forming apparatus receives an external force, the longer the horizontal distance between the grounding position of the caster provided on the side opposite the side where the force is received and the center-of-gravity position of the image forming apparatus, the more advantageous it is from the viewpoint of the stability of the image forming apparatus. On the other hand, the shorter the horizontal distance between the caster grounding position and the center-of-gravity position of the image forming apparatus, the more disadvantageous it is from the viewpoint of the stability of the image forming apparatus.
For example, when the image forming apparatus receives a pushing force from the outside, the caster rotates along the pushing direction. As a result, the caster faces the inner side of the apparatus, resulting in a short distance between the caster grounding position and the center-of-gravity position of the apparatus. That is, the caster is allowed to rotate to a more disadvantageous position as compared with the other, more stable rotational positions. Further, if the image forming apparatus is forcibly pushed with its wheels locked, each caster is allowed to rotate to a more disadvantageous position as compared with the other, more stable rotational positions.
In view of this, to lock such caster rotation, a construction is available in which it is not only possible to lock the caster wheel but also to lock the caster at an arbitrary rotational position through a single lever operation (See Japanese Patent Application Laid-Open No. 2005-278982). In this construction, a recessed engagement portion is provided in a support portion supporting the caster, and a protrusion provided on a lock member is engaged with the engagement portion, thereby locking the rotation of the caster. By thus locking the caster, it is possible to prevent the caster from rotating to a disadvantageous position as compared with the most stable position when the image forming apparatus is forcibly pushed, making it possible to achieve an improvement in terms of the safety of the image forming apparatus in the installed state.
Further, Japanese Patent Application Laid-Open No. 9-226952 and Japanese Patent Application Laid-Open No. 2006-175984 propose a caster lock mechanism in which it is not only possible to lock the wheel of the caster but also to lock the caster rotational position so as to cause it to face the outer side of the image forming apparatus. By thus restricting the caster rotational position to one where it faces the outer side of the image forming apparatus, it is possible to reliably lock the caster at an advantageous position in terms of safety.
In the image forming apparatus equipped with the conventional lock mechanism configured to simultaneously lock the wheel and lock the caster through a single lever operation, there may occur a case where the user strives to lock the wheel at a position where the recess inside the wheel and the protrusion of the lock member do not match each other. In this case, it is impossible to move the lever to the locking position even when the recessed engagement portion of the caster and the protrusion of the lock member match each other.
Thus, when performing lever operation, it is necessary for the user to simultaneously place in phase the protrusion of the lock member and the recess inside the wheel, and the protrusion of the lock member and the recessed engagement portion of the caster. However, it takes time and effort to simultaneously place in phase the protrusion of the lock member and the recess inside the wheel, and the protrusion of the lock member and the recessed engagement portion of the caster, resulting in deterioration in usability.